1. Field of the Invention
Embodiments of the invention relate generally to the field of photodetachment and purification of ion beams. More particularly, embodiments of the invention relate to methods and apparatus for efficient electron photodetachment and purification of negative ion beams.
2. Discussion of the Related Art
The Holifield Radioactive Ion Beam Facility (HRIBF) at the Oak Ridge National Laboratory is an isotope separator on-line (ISOL) facility providing high-quality radioactive ion beams (RIBs) for research in nuclear structure and nuclear astrophysics. At the Holifield Radioactive Ion Beam Facility (HRIBF), short-lived radioactive atoms are produced in selected target materials by nuclear reactions, ionized and mass-separated in a two stage magnetic separator before being injected into a 25 MV tandem electrostatic accelerator where the beam energies needed for research are obtained. The radioactive ion beams (RIBs) are used to study nuclear reactions of fundamental importance to research in nuclear astrophysics and nuclear structure. High beam intensity and purity are of crucial importance to many experiments. Unfortunately, there are many cases in which isobaric contaminants in the beam cannot be removed effectively by the magnetic separators. Significant yields of contaminant species compromise many experiments. Consequently, development of effective and efficient beam purification techniques has become a major focus at the Holifield Radioactive Ion Beam Facility (HRIBF) as well as other radioactive ion beam (RIB) facilities.
Tandem accelerators require negatively charged ions as input. There are a number of adjacent-Z species whose electron affinities are such that photodetachment can be used to suppress the unwanted negative ion species while leaving the species of interest intact. Examples of particular interest include suppressing the 56Co− component in a mixed 56Ni−+56Co− beam and the 17O− component in a mixed 17O−+17F− radioactive ion beams. Selectively removing the unwanted negative ion species by laser-induced photodetachment has been suggested for applications in accelerator mass spectrometry [1,2]. Selectively removing the unwanted negative ion species by laser-induced photodetachment has also been suggested for applications in isotope separator on-line radioactive ion beam production [3].
D. Berkovits, et al. [1,2] used a pulsed Nd:YAG laser of 10 ns pulse width and 30 Hz pulse repetition rate to selectively neutralize S and Co negative ions, while leaving the Cl and Ni negative ions unaffected. In their experiment, the negative ions were traveling with ˜100 keV energies, interacting with the laser beam over a distance of about 1.2 m. The overall degree of isobar suppression reported by D. Berkovits, et al. [1,2] was far from practically useful due to very short interaction time (a few micro seconds) between the pulsed laser beam and fast moving negative ion beams.
Meanwhile, gas-filled radio frequency (RF) quadrupole ion guides have been used extensively for ion beam cooling and bunching [4]. A RF quadrupole ion guide is a device in which ions with a selected mass/charge ratio are made to describe a stable path under the influence of a high frequency electrical field and are guided to pass through the device. When a buffer gas is introduced into the device, ions lose energy in collisions with the buffer gas molecules. With sufficient buffer gas pressure inside the ion guide, ion energy in both longitudinal and transverse directions can be reduced to the thermal energy of the buffer gas and the ion trajectories can be confined to a small region near the longitudinal axis of the device. Once cooled, the ions move at low velocity through the RF quadrupole under the influence of a modest longitudinal electrostatic field gradient.
Referring to FIG. 1, a Monte Carlo code has been used to simulate ion motions through gas-filled RF quadrupole ion guides [4]. FIG. 1 displays the calculated trajectories of negatively charged fluorine ions during transit through a 10 cm long RF quadrupole ion guide filled with helium at a gas pressure of 1.33 Pa. The negative ions enter the RF quadrupole with an initial energy of 40 eV. As noted, collisional cooling and focusing effects are clearly observed in the Monte Carlo simulations.
Heretofore, the requirement(s) of effective and efficient beam purification techniques referred to above have not been fully met. What is needed is a solution that simultaneously provides both effective and efficient beam purification.